minor: change transformers args to list

[hadysata]

The args in the asset-transformation section should be of type list of strings, not a single string as the documentation shows. Running the code as in the docs will result in the following issue:

Error detected in pubspec.yaml:
Unable to parse assets section.
In transformers section of asset "assets/logo.svg": In args section of transformer using package "vector_graphics_compiler": Expected args to be a list of String, but got --tessellate --font-size=14 (String).
Please correct the pubspec.yaml file

This PR convert the args section in the docs to list.

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[domesticmouse]

/gcbrun

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Visit the preview URL for this PR (updated for commit 005a165ab477c09b8dfd45d6727b1254c0ff2b48):

https://flutter-docs-prod--pr10622-fix-args-to-list-t9pneitv.web.app